The Greatest Common Factor (GCF or HCF) is the largest number that divides evenly into two or more numbers. The process of finding the GCF is to list all of the factors of each number and find the largest number that is a factor of all of the numbers.
 | = | least common multiple |
 | = | greatest common divisor |
 | = | non-zero integers |
For example, the GCF of 12 and 18 is 6 because 6 is the largest number that is a factor of both 12 and 18.
Example: Find the GCF of 15 and 30.
The factors of 15 are 1, 3, 5, and 15. The factors of 30 are 1, 2, 3, 5, 10, and 15. The largest number that is a factor of both 15 and 30 is 15.